1. Field of the Invention
The present invention relates generally to printed circuit boards (PCB) populated with power supplies used to furnish DC voltages and DC currents to integrated circuits, and more specifically to a method and system for measuring and/or balancing DC currents flowing from current sources to integrated circuits.
2. Background of the Invention
Current sharing allows a distribution of load current amongst a number of parallel DC to DC converters to increase the current and power delivery to a load by having each of the converters contribute some component of the total current required. Current sharing improves the system transient response by spreading the load over all paralleled converters and improves reliability by reducing their individual power dissipation. With current sharing, each parallel converter contributes approximately a 1/N+1 portion of the load current, when there are a total of N+1 paralleled converters with a minimum of N converters required for providing maximum load current. One converter does not have to contribute a disproportionate amount or reach a current limit before other parallel converters contribute. If some of the paralleled converters should fail, the load current is distributed among the remaining converters and the system continues to operate.
Current sharing can be based on the output current of each of the DC to DC converters or can be based on the current carried by each of the switching devices in each of the converters. When output current or load current is monitored, an output sense resistor is used in each of the parallel regulators. The sense resistor dissipates considerable amounts of power when the converters provide high output currents.
Most commercially available DC converters do not provide current control or current monitoring. Therefore, external sensor circuits and external current sharing controllers are required. Generally, a discrete resistor is used to measure the current. FIGS. 1A and 1B illustrate the use of a discrete resistor to measure current to control DC to DC converters for efficient current sharing. FIG. 1A is a partial cross-sectional view of a printed circuit board 100 and FIG. 1B is a partial plan view of the external upper layer. PCB 100 comprises internal and external conductive layers, usually made of copper, in which wiring tracks may be designed and drawn. PCB 100 includes the conductive plane 105 that is part of the power plane grid implemented in one of the internal conductive layers. The conductive tracks 110 and 115 are part of the external conductive upper layer. Conductive vias 125 are used to electrically connect power plane 105 and conductive track 115, as illustrated. Distributing current through a sufficient number of vias 125 reduces the voltage drop at any given node on the board. Once conductive tracks of the external layers have been designed, PCB 100 is partially covered with an insulative solder mask protective coating 120 to protect the board and to avoid shorts. The parts that are not covered with insulative material are plated so that electronic components may be soldered to the PCB 100. A Pin-In-Hole (PIH) DC to DC converter 130 is connected to conductive track 110 of PCB 100 with pin 135 and a discrete resistor 140 is connected to conductive tracks 110 and 115 as illustrated. An electronic component 145 having a pin 150 connected to the power plane 105 is also shown in FIG. 1.
FIG. 1B illustrates the external upper layer comprising conductive tracks 110 and 115. Each conductive track 110 and 115 end with a pad referred to as 155-1 and 155-2, respectively, on which discrete resistor 140 is soldered. The footprint of discrete resistor 140 is represented by a dotted line. Two further conductive tracks 160-1 and 160-2 are connected to the pads 155-1 and 155-2, respectively, and to a current sharing controller, not represented for clarity. While a single internal power plane is shown on FIG. 1A, it is understood that a PCB generally comprises several internal power and wiring planes, in a multi-layered PCB. The number of internal power planes depends upon the power required and the number of DC to DC converters mounted on the PCB. Distributing current through several internal power planes reduces the voltage drop and therefore current loss.
A standard solution for measuring current between DC to DC converter 130 and power plane 105 consists in removing discrete resistor 140 so as to solder a copper wire between pads 155-1 and 155-2 and to use a current clamp.
To achieve a lower resistor value, two discrete resistors in close proximity may be arranged in parallel. However, using two discrete resistors per converter may increase the dedicated area for such components and the cost of PCB design.
A need exists in the art for a method and system for measuring current at the output of a DC to DC converter and for optimizing current sharing between several DC to DC converters employed on a PCB such that the problems described above are overcome. Particular scrutiny must be given to the additional cost and area required to implement a satisfactory PCB solution.